Steel sheet for crown cap, manufacturing method therefor, and crown cap

ABSTRACT

A steel sheet for a crown cap has sufficient strength and formability even when the thickness thereof is reduced for use, and has a composition containing, in percent by mass, C: 0.010% to 0.025%, Si: 0.10% or less, Mn: 0.05% to 0.50%, P: 0.050% or less, S: 0.005% to 0.050%, Al: 0.020% to 0.070%, N: less than 0.0040%, and the balance being Fe and inevitable impurities, wherein yield strength after heat treatment at 210° C. for 15 minutes is as follows: the yield strength is 550 MPa or more in a rolling direction, and the yield strength in a direction 45° from the rolling direction in a rolling plane is equal to or less than the average of the yield strength in the rolling direction and the yield strength in a direction 90° from the rolling direction in the rolling plane.

TECHNICAL FIELD

This disclosure relates to a steel sheet used as a material for a crown cap serving as a cap for a glass bottle, a method of manufacturing the same, and a crown cap.

BACKGROUND

Glass bottles have been widely used for a long time as containers for beverages such as soft drinks and alcoholic drinks. A cap made of metal, which is referred to as a crown cap, is generally used for a narrow-mouthed glass bottle. In general, crown caps are manufactured by press forming by using a thin steel sheet as a material. A crown cap includes a disk-shaped portion that covers the mouth of a bottle and a pleated portion disposed on the periphery thereof, and by crimping the pleated portion around the mouth of the bottle, the bottle is hermetically sealed.

The characteristics required for a thin steel sheet used as a material for crown caps include strength and formability. Bottles provided with a crown cap are often filled with contents that cause an internal pressure such as beer or a carbonated beverage. The material is required to have a strength such that, even when the internal pressure is increased because of a change in temperature or the like, the seal of the bottle is not broken by deformation of the crown cap. Furthermore, even if the strength of the material is sufficient, when the material has poor formability, the shape of pleats may become non-uniform, and sufficient sealing performance cannot be obtained even when the pleated portion is crimped around the mouth of a bottle.

As the thin steel sheet used as a material for crown caps, an SR (Single Reduced) steel sheet is mainly used. An SR steel sheet is produced by reducing the thickness of a steel sheet by cold rolling, and then annealing the steel sheet, followed by temper rolling. In existing techniques, since the thickness of the material for crown caps is 0.20 mm or more, it is possible to secure sufficient strength and formability by employing an SR sheet made from mild steel used for cans of food and drinks.

However, in recent years, there has been an increased demand for a reduction in the thickness of the material for crown caps, as in steel sheets for cans, for the purpose of cost reduction. When the thickness of the material for crown caps is less than 0.20 mm, the strength of the existing SR steel sheet is insufficient. To ensure strength, it is possible to use a DR (Double Reduced) steel sheet that has been subjected to secondary cold rolling after annealing. However, when the secondary cold rolling reduction is increased, formability is degraded, resulting in poor sealing of bottles.

Under the circumstances described above, to obtain a steel sheet having excellent strength and formability, the following techniques have been proposed.

Japanese Unexamined Patent Application Publication No. 2001-49383 discloses an ultrathin soft steel sheet for a container excellent in can strength and can formability, containing, in percent by weight, N: 0.0040 to 0.0300% and Al: 0.005 to 0.080%, characterized in that the 0.2% proof stress is 430 MPa or less, the total elongation is 15 to 40%, and the internal friction Q⁻¹ is 0.0010 or more.

Japanese Unexamined Patent Application Publication No. 2013-28842 discloses a high-strength high-workability steel sheet for a can, containing, in percent by mass, C: 0.001 to 0.080%, Si: 0.003 to 0.100%, Mn: 0.10 to 0.80%, P: 0.001 to 0.100%, S: 0.001 to 0.020%, Al: 0.005 to 0.100%, N: 0.0050 to 0.0150%, and B: 0.0002 to 0.0050%, characterized by including, in area fraction, 0.01 to 1.00% of crystal grains whose elongation rate is 5.0 or more in a cross section in the rolling direction.

However, the existing techniques have problems as described below.

The steel sheet described in JP '383 is soft and contains a large amount of N. Therefore, to obtain the required strength, it is necessary to increase the secondary cold rolling reduction. When the secondary cold rolling reduction is increased, anisotropy is also increased, and formability is impaired.

The steel sheet described in JP '842 has a large N content as in the steel sheet described in JP '383. Therefore, it is difficult to achieve both strength and workability required for the material for crown caps.

It could therefore be helpful to provide a steel sheet for a crown cap having sufficient strength and formability even when the thickness thereof is reduced for use, a manufacturing method therefor, and a crown cap.

SUMMARY

We found that by controlling the steel composition, hot rolling conditions, annealing conditions, and secondary cold rolling conditions (DR conditions), it is possible to obtain a steel sheet for a crown cap having sufficient strength and formability.

We thus provide:

[1] A steel sheet for a crown cap comprising:

a composition containing, in percent by mass, C: 0.010% to 0.025%, Si: 0.10% or less, Mn: 0.05% to 0.50%, P: 0.050% or less, S: 0.005% to 0.050%, Al: 0.020% to 0.070%, N: less than 0.0040%, and the balance being Fe and inevitable impurities,

regarding yield strength after heat treatment at 210° C. for 15 minutes,

a yield strength of 550 MPa or more in a rolling direction, and

a yield strength in a direction 45° from the rolling direction in a rolling plane equal to or less than an average of the yield strength in the rolling direction and a yield strength in a direction 90° from the rolling direction in the rolling plane.

[2] A method of manufacturing the steel sheet for a crown cap according to [1], including:

a hot rolling step of hot rolling a slab and coiling the hot rolled steel sheet at a coiling temperature of 530° C. to 590° C.;

a primary cold rolling step of cold rolling the hot rolled steel sheet after the hot rolling step;

an annealing step of annealing the cold rolled steel sheet at an annealing temperature of 650° C. to 720° C. after the primary cold rolling step; and

a secondary cold rolling step of performing secondary cold rolling with a rolling reduction of 25% to 40% after the annealing step.

[3] A crown cap formed of the steel sheet for a crown cap according to [1]. [4] A method of manufacturing a steel sheet for a crown cap including:

a hot rolling step of hot rolling a slab having a composition containing, in percent by mass, C: 0.010% to 0.025%, Si: 0.10% or less, Mn: 0.05% to 0.50%, P: 0.050% or less, S: 0.005% to 0.050%, Al: 0.020% to 0.070%, N: less than 0.0040%, and the balance being Fe and inevitable impurities, and coiling the hot rolled steel sheet at a coiling temperature of 530° C. to 590° C.;

a primary cold rolling step of cold rolling the hot rolled steel sheet after the hot rolling step;

an annealing step of annealing the cold rolled steel sheet at an annealing temperature of 650° C. to 720° C. after the primary cold rolling step; and

a secondary cold rolling step of performing secondary cold rolling with a rolling reduction of 25% to 40% after the annealing step,

the manufactured steel sheet having, regarding yield strength after heat treatment at 210° C. for 15 minutes, a yield strength in a rolling direction is 550 MPa or more, and a yield strength in a direction 45° from the rolling direction in a rolling plane is equal to or less than the average of the yield strength in the rolling direction and a yield strength in a direction 90° from the rolling direction in the rolling plane.

It is thus possible to obtain a steel sheet for a crown cap having sufficient strength and formability even when the thickness thereof is reduced for use. It is possible to achieve both strength and crown cap formability of a steel sheet, and a reduction in the thickness of a crown cap can be realized.

DETAILED DESCRIPTION

Our steel sheets, methods and crown caps will be described in detail below. Unless otherwise specified, “%” means percent by mass.

Our steel sheet has a specific composition, and regarding yield strength after heat treatment at 210° C. for 15 minutes, the yield strength in a rolling direction is 550 MPa or more, and the yield strength in a direction 45° from the rolling direction in a rolling plane is equal to or less than the average of the yield strength in the rolling direction and the yield strength in a direction 90° from the rolling direction in the rolling plane. As a result, it is possible to obtain a crown cap having sufficient strength and formability and whose thickness can be reduced for use.

Composition

The composition will be described.

C: 0.010% to 0.025%

C is an element that contributes to achieving both strength and workability when its content is in a controlled range. When the C content is less than 0.010%, the amount of strengthening due to solute C is small and, therefore, strength becomes insufficient. On the other hand, when the C content exceeds 0.025%, the shape of pleats of a formed crown cap becomes non-uniform, resulting in shape defects. Therefore, the C content is 0.010% to 0.025%.

Si: 0.10% or less

When the Si content is excessively large, formability is adversely affected. Accordingly, the Si content exceeding 0.10% is not desirable. Therefore, the Si content is 0.10% or less. From the viewpoint of improvement in the strength of the steel sheet, the Si content is preferably 0.02% to 0.10%.

Mn: 0.05% to 0.50%

When the Mn content falls below 0.05%, even when the S content is decreased, it becomes difficult to avoid hot brittleness, resulting in problems such as surface cracking during continuous casting. On the other hand, the Mn content exceeding 0.50% adversely affects formability, similarly to Si. Therefore, the Mn content is 0.05% to 0.50%.

P: 0.050% or less

When the P content exceeds 0.050%, steel is hardened, and corrosion resistance is decreased. Therefore, the P content is 0.050% or less.

S: 0.005% to 0.050%

S binds to Mn to form MnS in steel and precipitates a large amount of MnS, thereby degrading the hot ductility of steel. When the S content exceeds 0.050%, this effect becomes noticeable. On the other hand, to set the S content to be less than 0.005%, the desulfurization cost becomes excessively high. Therefore, the S content is 0.005% to 0.050%

Al: 0.020% to 0.070%

Al is an element added as a deoxidizer. Furthermore, Al forms AlN with N in steel to decrease solute N in steel. When the Al content is less than 0.020%, the effect as a deoxidizer is insufficient, resulting in solidification defects. On the other hand, when the extent of secondary cold rolling is large, a large Al content will cause degradation in formability. When the Al content exceeds 0.070%, the shape of pleats becomes non-uniform during formation of a crown cap, resulting in shape defects. Therefore, the Al content is 0.020% to 0.070%.

N: less than 0.0040%

When the N content is 0.0040% or more, the steel sheet is hardened, and formability is degraded. Therefore, the N content is less than 0.0040%, and preferably 0.0035% or less.

The balance other than the essential components described above includes iron and inevitable impurities.

Mechanical Properties

Mechanical properties of a steel sheet for a crown cap will be described below.

A steel sheet for a crown cap is required to have a strength such that the crown cap is not removed under the influence of an internal pressure of the bottle. For this reason, in existing techniques, the thickness of a steel sheet for a crown cap is 0.20 mm or more. However, there has been an increased demand for a reduction in the thickness. When the thickness is reduced to less than 0.20 mm, a larger strength than that of existing materials is required. When the yield strength in a rolling direction of a steel sheet is less than 550 MPa, it is not possible to impart a sufficient strength to a crown cap whose thickness has been reduced, and the pressure resistance becomes insufficient. Therefore, the yield strength in the rolling direction is 550 MPa or more.

Furthermore, in general, in a DR steel sheet, the yield strength differs depending on the direction in the rolling plane. When the yield strength in a direction 45° from the rolling direction is more than the average of the yield strength in the rolling direction and the yield strength in a direction 90° from the rolling direction, formability is degraded. Therefore, the yield strength in a direction 45° from the rolling direction in a rolling plane is equal to or less than the average of the yield strength in the rolling direction and the yield strength in a direction 90° from the rolling direction in the rolling plane. That is, the difference obtained by subtracting the yield strength in a direction 45° from the rolling direction in the rolling plane from the average of the yield strength in the rolling direction and the yield strength in a direction 90° from the rolling direction in the rolling plane is 0 MPa or more, and preferably 10 to 25 MPa.

It is possible to manufacture a steel sheet having the yield strength described above by a manufacturing method which will be described later.

Furthermore, a crown cap is formed, often after a steel sheet has been subjected to baking finish and, therefore, it is necessary to evaluate the quality of the material after treatment corresponding to baking finish. Accordingly, the yield strength is measured after heat treatment corresponding to baking finish at 210° C. for 15 minutes, and the tensile testing method for metallic materials according to “JIS Z 2241” can be applied thereto.

Method of Manufacturing Steel Sheet for Crown Cap

An example of a method of manufacturing a steel sheet for a crown cap will be described below.

Our method of manufacturing a steel sheet for a crown cap includes a hot rolling step in which a steel slab having the composition described above is subjected to hot rolling, followed by coiling at a coiling temperature of 530° C. to 590° C.; a primary cold rolling step in which cold rolling is performed after the hot rolling step; an annealing step in which annealing is performed at an annealing temperature of 650° C. to 720° C. after the primary cold rolling step; and a secondary cold rolling step in which secondary cold rolling is performed with a rolling reduction of 25% to 40% after the annealing step. These steps will be described below.

Hot Rolling Step

Molten steel is adjusted to have the chemical composition described above by a known method using a converter or the like, and is formed into a slab by a continuous casting method. Subsequently, the steel slab is subjected to rough rolling. Although the rough rolling method is not particularly limited, the slab heating temperature is preferably 1,200° C. or higher. Then, finish rolling is performed. The finish rolling temperature is preferably 850° C. or higher from the standpoint of stability of the rolling load. The term “finish rolling temperature” refers to the temperature of the sheet when it enters the last stand of a finish rolling mill. On the other hand, when the finish rolling temperature is increased more than necessary, manufacturing of a thin steel sheet may become difficult in some cases. That is, when the thickness of the sheet is small since the decrease in the temperature of the sheet during rolling is large, it is difficult to perform finish rolling while maintaining the high temperature of the sheet, thus being uncontrollable. Therefore, the finishing temperature is preferably 850° C. to 900° C.

When the coiling temperature in the hot rolling step is lower than 530° C., to perform an operation without impairing efficiency, it is necessary to decrease the finish rolling temperature accordingly, which is inappropriate. On the other hand, when the coiling temperature exceeds 590° C., the amount of AlN that precipitates after coiling becomes excessively large, leading to a decrease in the grain size after annealing and resulting in a degradation in formability. Therefore, the coiling temperature is 530° C. to 590° C., and preferably 540° C. to 580° C.

Primary Cold Rolling Step

It is preferable to remove surface scale before the primary cold rolling step. The method of removing surface scale is not particularly limited, and various common methods such as pickling and physical removal can be used. Surface scale can be suitably removed by pickling. The picking conditions are not particularly limited, and pickling may be performed in the usual manner.

The rolling reduction in the primary cold rolling is preferably 85% or more to manufacture an ultrathin material. However, when the rolling reduction is excessively increased, the load on the rolling machine becomes excessively large, and it may become difficult to perform rolling in some cases. Therefore, the rolling reduction is preferably 94% or less.

Annealing Step

When the annealing temperature is higher than 720° C., trouble during passing of the sheet such as heat buckling is likely to occur during continuous annealing, which is not desirable. When the annealing temperature is lower than 650° C., recrystallization becomes imperfect, resulting in non-uniform quality of the material. Therefore, the annealing temperature is 650° C. to 720° C. Furthermore, the soaking period in the annealing step is not particularly limited, but is preferably 10 seconds or more to surely obtain a recrystallization structure, and is preferably 50 seconds or less to prevent excessive grain growth.

Secondary Cold Rolling (DR Rolling) Step

The strength of the annealed steel sheet is increased by secondary cold rolling. When the rolling reduction in the secondary cold rolling is less than 25%, it is not possible to obtain strength sufficient to ensure the pressure resistance of a crown cap. Furthermore, when the rolling reduction in the secondary cold rolling exceeds 40%, the difference of the yield strength in a direction 45° from the rolling direction in the rolling plane from the average of the yield strength in the rolling direction and the yield strength in a direction 90° from the rolling direction in the rolling plane is positively increased, resulting in degradation in formability. Therefore, the rolling reduction in the secondary cold rolling is 25% to 40%.

A high-strength steel sheet is obtained by the method described above. Even when the resulting steel sheet is subjected to surface treatment such as plating or chemical conversion treatment, the advantageous effects are not impaired.

Examples

Steels having the compositions shown Table 1 and containing the balance being Fe and inevitable impurities were refined by a converter, and steel slabs were obtained by continuous casting. The resulting steel slabs were each heated to 1,250° C., then hot-rolled at a rolling start temperature of 1,150° C. and at a finish rolling temperature of 860° C., and coiled at the coiling temperature shown in Table 2. Next, after scale was removed by pickling, primary cold rolling was performed with the primary cold rolling reduction shown in Table 2, annealing was performed in a continuous annealing furnace at the annealing temperature shown in Table 2, and secondary cold rolling (DR rolling) was performed with the secondary cold rolling reduction shown in Table 2 to obtain steel sheets (levels 1 to 9) with a thickness of 0.15 to 0.18 mm. The resulting steel sheets were Cr-plated on both surfaces with a coating weight of 100 mg/m² per surface and, thereby, tin-free steel sheets were obtained.

Characteristics evaluation was made on the steel sheets thus obtained by the methods described below.

Yield Strength

After conducting heat treatment corresponding to baking finish at 210° C. for 15 minutes, a tensile test was carried out. The tensile test was performed, using a JIS No. 5 tensile test piece, in accordance with “JIS Z 2241” to measure the yield strength in the rolling direction, the yield strength in a direction 45° from the rolling direction in the rolling plane, and the yield strength in a direction 90° from the rolling direction in the rolling plane.

Crown Cap Formability

Using each of the resulting steel sheets, a crown cap was formed, and crown cap formability was evaluated. A circular blank with a diameter of 37 mm was used and formed by press working into a size (outside diameter: 32.1 mm, height: 6.5 mm, number of pleats: 21) of a type 3 crown cap according to “JIS S 9017” (withdrawn standard). The evaluation was visually performed. When the pleats all had a uniform size, they were evaluated as being very good (⊙), when the pleats had a substantially uniform size they were evaluated as being good (◯), and when the pleats had a non-uniform size they were evaluated as being poor (×). In the visual determination, when the maximum value of pleat width (breadth) was 1.5 times or more the minimum value was determined to be non-uniform.

Pressure Test Using Formed Crown Cap

Crown caps were formed by the same method as that described above. A vinyl chloride liner was formed inside each of the crown caps. A commercially available beer bottle was closed with the crown cap, and the internal pressure at which the crown cap was removed was measured by using a Secure Seal Tester manufactured by Glassline Corporation.

When the pressure resistance was equal to or more than that of the existing crown cap they were evaluated as being good (◯), and when the pressure resistance did not reach that of the existing crown cap they were evaluated as being poor (×).

The results thus obtained are shown in Table 3.

TABLE 1 (mass %) C Si Mn P S Al N Level 1 0.019 0.01 0.22 0.015 0.010 0.057 0.0034 Level 2 0.025 0.01 0.30 0.019 0.013 0.035 0.0029 Level 3 0.011 0.02 0.25 0.016 0.020 0.031 0.0030 Level 4 0.020 0.01 0.48 0.021 0.015 0.046 0.0031 Level 5 0.022 0.03 0.41 0.018 0.019 0.069 0.0027 Level 6 0.019 0.01 0.28 0.020 0.020 0.061 0.0038 Level 7 0.009 0.02 0.24 0.012 0.010 0.040 0.0030 Level 8 0.015 0.01 0.35 0.013 0.015 0.046 0.0032 Level 9 0.021 0.03 0.40 0.021 0.011 0.031 0.0033

TABLE 2 Hot rolling Primary Secondary coiling Thickness cold rolling Annealing cold rolling Thickness temperature of hot-rolled reduction temperature reduction of finished (° C.) sheet (mm) (%) (° C.) (%) sheet (mm) Remarks Level 1 570 2.5 90 680 30 0.18 Example Level 2 540 2.5 88 660 40 0.18 Example Level 3 580 2.8 90 700 35 0.18 Example Level 4 530 2.5 92 650 25 0.15 Example Level 5 590 2.5 90 690 30 0.18 Example Level 6 550 2.8 90 720 35 0.18 Example Level 7 560 2.5 88 650 40 0.18 Comparative Example Level 8 570 2.8 90 640 35 0.18 Comparative Example Level 9 560 2.5 92 670 20 0.16 Comparative Example

TABLE 3 Average of yield strength in rolling Yield direction and Yield strength strength in yield strength in in direction 45° rolling direction 90°\ from rolling Crown direction from rolling direction (MPa) {circle around (1)}-{circle around (2)} cap Pressure (MPa) direction (MPa) {circle around (1)} {circle around (2)} (MPa) formability resistance Remarks Level 1 610 632 619 13 ⊙ ◯ Example Level 2 628 650 640 10 ◯ ◯ Example Level 3 621 645 630 15 ⊙ ◯ Example Level 4 579 602 592 10 ◯ ◯ Example Level 5 604 622 616 6 ◯ ◯ Example Level 6 617 643 631 12 ◯ ◯ Example Level 7 545 563 556 7 ◯ X Comparative Example Level 8 625 644 652 −8 X X Comparative Example Level 9 543 557 555 2 X X Comparative Example

As shown in Table 3, in the steel sheets of levels 1 to 6 which are our examples, the yield strength in the rolling direction is 550 MPa, the yield strength in a direction 45° from the rolling direction in the rolling plane is equal to or less than the average of the yield strength in the rolling direction and the yield strength in a direction 90° from the rolling direction in the rolling plane, and both the crown cap formability and the pressure resistance are satisfactory.

On the other hand, in the steel sheet of level 7 which is a comparative example, since the C content is excessively low, the yield strength in the rolling direction is less than 550 MPa, and the pressure resistance is insufficient. In the steel sheet of level 8 which is a comparative example, since the annealing temperature is excessively low, the yield strength in a direction 45° from the rolling direction exceeds the average of the yield strength in the rolling direction and the yield strength in a direction 90° from the rolling direction in the rolling plane, and the crown cap formability is poor. Since the shape of the crown cap is poor, sealing performance is insufficient, and the pressure resistance is low. In the steel sheet of level 9 which is a comparative example, since the secondary cold rolling reduction is excessively small, the yield strength in the rolling direction is less than 550 MPa, the crown cap formability is poor, and the pressure resistance is insufficient. 

1-4. (canceled)
 5. A steel sheet for a crown cap comprising: a composition containing, in percent by mass, C: 0.010% to 0.025%, Si: 0.10% or less, Mn: 0.05% to 0.50%, P: 0.050% or less, S: 0.005% to 0.050%, Al: 0.020% to 0.070%, N: less than 0.0040%, and the balance being Fe and inevitable impurities, the steel sheet having a yield strength after heat treatment at 210° C. for 15 minutes, wherein 1) and 2): 1) yield strength is 550 MPa or more in a rolling direction, and 2) yield strength in a direction 45° from the rolling direction in a rolling plane is equal to or less than an average of the yield strength in the rolling direction and a yield strength in a direction 90° from the rolling direction in the rolling plane.
 6. A method of manufacturing the steel sheet for a crown cap according to claim 5, comprising: a hot rolling step of hot rolling a slab and coiling the hot rolled steel sheet at a coiling temperature of 530° C. to 590° C.; a primary cold rolling step of cold rolling the hot rolled steel sheet after the hot rolling step; an annealing step of annealing the cold rolled steel sheet at an annealing temperature of 650° C. to 720° C. after the primary cold rolling step; and a secondary cold rolling step of performing secondary cold rolling with a rolling reduction of 25% to 40% after the annealing step.
 7. A crown cap comprising the steel sheet for a crown cap according to claim
 5. 8. A method of manufacturing a steel sheet for a crown cap comprising: a hot rolling step of hot rolling a slab having a composition containing, in percent by mass, C: 0.010% to 0.025%, Si: 0.10% or less, Mn: 0.05% to 0.50%, P: 0.050% or less, S: 0.005% to 0.050%, Al: 0.020% to 0.070%, N: less than 0.0040%, and the balance being Fe and inevitable impurities, and coiling the hot rolled steel sheet at a coiling temperature of 530° C. to 590° C.; a primary cold rolling step of cold rolling the hot rolled steel sheet after the hot rolling step; an annealing step of annealing the cold rolled steel sheet at an annealing temperature of 650° C. to 720° C. after the primary cold rolling step; and a secondary cold rolling step of performing secondary cold rolling with a rolling reduction of 25% to 40% after the annealing step, the manufactured steel sheet having a yield strength after heat treatment at 210° C. for 15 minutes, wherein 1) a yield strength of 550 MPa or more in a rolling direction, and 2) a yield strength in a direction 45° from the rolling direction in a rolling plane being equal to or less than an average of the yield strength in the rolling direction and a yield strength in a direction 90° from the rolling direction in the rolling plane. 